When you feel comfortable enough to want to help tackle issues by trying to create a patch to fix an issue, you can start by looking at the "easy" issues. These issues should be ones where it should take no longer than a day or weekend to fix. But because the "easy" classification is typically done at triage time it can turn out to be inaccurate, so do feel free to leave a comment if you think the classification no longer applies.
For the truly adventurous looking for a challenge, you can look for issues that are not considered easy and try to fix those. It must be warned, though, that it is quite possible that a bug that has been left open has been left into that state because of the difficulty compared to the benefit of the fix. It could also still be open because no consensus has been reached on how to fix the issue (although having a patch that proposes a fix can turn the tides of the discussion to help bring it to a close). Regardless of why the issue is open, you can also always provide useful comments if you do attempt a fix, successful or not.